Spinal Cord

Spinal Cord. The spinal cord is a cylinder of soft nervous tissue which extends from the medulla oblongata to the first lumbar vertebra, being contained in the spinal canal of the vertebral column. It is ensheathed in membranous structures, the attachments of which serve to maintain it in position, and from its lower extremity a narrow cord, the filum terminale, depends. The spinal cord gives off 31 pairs of nerves [NERVE], and it is bulged in the situations from which the nerves which form the brachial and lumbar plexuses emanate; these points of swelling are spoken of as the cervical and lumbar enlargements. The cord consists of two lateral halves, which are almost completely separated from one another by the anterior and posterior median fissures; the former of these is an actual cleft.; the latter is constituted by a penetration into the substance of the cord of a partition formed by one of the enveloping membranes, the pia mater. The cord consists of white and grey matter, the former constituting the outer part of the cord, the latter lying internally and being disposed in a shape which has been compared to that of the letter H. The grey matter thus consists of two lateral halves united by a median band, in the centre of which is situated the central canal of the cord. That portion of the median band which lies in front of the central canal is called the anterior commissure, and that portion which lies behind it is called the posterior commissure. The extremities of each lateral arm of the grey matter are called horns, anterior and posterior respectively. The anterior horn presents a rounded bulbous enlargement, from which a number of strands of nerve fibres pass outwards at certain points and form the anterior roots of each of the spinal nerves; many of these nerve-fibres communicate with the large ganglionic nerve-cells, which are disposed in a number of more or less definitely localised clusters in the anterior horn of the grey matter of the cord. The postorior horns are prolonged outwards in the form of strands of nerve fibres which pass into the posterior roots of the several spinal nerves. By the emergence of nerve fibres from the grey matter of the cord to form the roots of the spinal nerves, the white matter lying superficially to the central core of grey matter is split up into three columns, anterior, lateral, and posterior, as they are termed. Examined microscopically, the grey matter consists of nerve-cells, nerve-fibres, and a network (neuroglia); it is richly-supplied with blood-vessels. The white matter contains a supporting framework of neuroglia, but is mainly composed of white medullated nerve fibres, which are disposed longitudinally in the substance of the cord. An account of the connections which are established by these columns of white fibres and of their association with the transmission of motor impulses is given under the heading PARALYSIS. The motor impulses, after passing downwards in the "motor path" [PARALYSIS], emerge from the cord along the anterior roots of the spinal nerves. The posterior roots are largely concerned with the transmission of sensory impulses, which reach them after travelling from the periphery along the nerve trunks, and are conveyed through the posterior roots to the grey matter of the cord.

Diseases of the Spinal Cord. When disease affects the spinal cord, it is apt to be limited to certain regions, and in association with the locality involved special symptoms occur. In the disease known as infantile spinal paralysis, the ganglion cells in the anterior horns of the grey matter are particularly affected. Certain degenerations involve particular longitudinally-disposed strands of white matter; for instance, in locomotor ataxia (q.v.) the posterior columns of the cord are specially implicated in the degenerative process. Degeneration of parts of the lateral columns of the cord occurs in association with affection of those portions of the anterior columns which adjoin the anterior median fissure. Such degeneration is often spoken of as "descending degeneration," as it usually progresses from above downwards, and is attributable to disease affecting some portion of the motor tract, and the nerve-fibres, which travel along the cord in the situations indicated, are thus cut off from their normal connection with the upper parts of the system of nervous structures, and therefore, undergo degeneratile change. [PARALYSIS.] The membranes of the cord are sometimes affected by inflammation (spinal meningitis), and a diffuse inflammation of the substance of the cord (myelitis) is in rare instances met with. [MYELITIS.] Angular curvature of the spine (Pott's curvature), which is a disease produced by the involvement of the vertebrae by tubercular disease, with eating away of their substance and the formation of collections of matter, usually produces symptoms attributable to the pressure which is exerted under the abnormal conditions upon the portions of the spinal cord which are enclosed within the diseased structures. Spina bifida is a form of congenital malformation in which there is a sac-like protrusion of the membranes of the cord through a cleft left in the wall of the enclosing bony canal, the non-closure of the cleft being the result of imperfect development.